3D AVO Research Articles

Value of PS Waves for 3D and 4D AVO Inversion

Value of PS Waves for 3D and 4D AVO Inversion

Prestack 3D and 4D seismic inversion for reservoir static and dynamic properties

We present a successful case study in which prestack 3D and 4D simultaneous AVO inversion is used in conjunction with rock-physics analysis to estimate saturation and pressure changes in a West Africa brownfield using time-lapse (4D) seismic as input. We show that, in 4D seismic, there can be many competing production effects that can be difficult to disentangle using traditional 4D interpretation methods, such as amplitude differences between the base and monitors. This begs the need for a more sophisticated approach to decouple these competing effects, such as the use of prestack simultaneous 3D and 4D inversions. Multiple substack seismic data are used to estimate a variety of 3D and 4D petroelastic attributes for mapping static and dynamic reservoir properties with the primary objective of influencing the continuous infill drilling and the overall reservoir management strategy. Facies-specific low-frequency models were used as priors for the 3D inversion, while velocity changes from time-lapse time shifts were used as priors for the 4D inversion. We also demonstrate the use of rock-physics templates coupled with a lithology-specific Gassmann fluid-substitution method to establish a nonlinear regression-based rock-physics model that obeys bound theory from classical rock physics and honors single and multimineral fluid-substitution theory. The resulting templates, when integrated with the prestack AVO-inversion technique, produce a set of attributes that accurately explain the time-lapse production effects observed on seismic.

Is there a benefit to throwing the kitchen sink at geotechnical studies in an exploration phase?

In short, yes. This case study illustrates that the application of a thorough geotechnical workflow incorporating many new and advanced techniques can assist in exploration business case decision making. Is an exploration drilling decision made lightly? A workflow incorporating 3D seismic processing, AVO inversion and stratigraphic framework studies involving high-resolution biostratigraphic and chemostratigraphic analyses was used to assess the prospectivity of an exploration permit near giant gas fields in the offshore Northern Carnarvon Basin. The primary reservoir is the prolific Triassic Mungaroo Formation fluvio-deltaic sediments, and secondary reservoirs include mid-Jurassic marine sands. 3D seismic reprocessing xcombined a newly acquired broadband seismic dataset into a multi-survey multi-azimuth PSDM volume that conditioned data for input to an AVO inversion. New petrophysics and rock physics analysis and modelling on regional well data were then calibrated with the AVO inversion to statistically derive lithology and fluid prediction volumes. These data were used in conjunction with reservoir paleo-stratigraphy studies to derive a subsurface model for reservoir distribution and hydrocarbon prediction. A two-stage risking process was applied to each prospect that objectively applied risk based on the seismic amplitudes. This enabled a more accurate risked-volume assessment, combined with the ability to assess a prospect portfolio covering different plays. The resultant interpretation identified issues with interpretations made on vintage data that would not have been easily identified without undertaking these studies. The integration of these assessments resulted in an unfavourable exploration drilling business case and a decision not to renew the permit.

Benefiting from 3D AVO by using adaptive supergathers

3D surface seismic data have many useful features (e.g., fine sampling, better resolution) that, coupled with the fact that 3D prestack time or depth migration yields accurate imaging of subsurface features, are good reasons to conclude that performing AVO analysis on 3D data will be beneficial as it is on 2D data.

Application of 3D AVO interpretation technique to lithological reservoir in the Hongze Area

In the Hongze Area, the reservoirs vary rapidly laterally and are controlled by many factors, such as structure, lithology, oil source, and so on. S-wave well log curves are calculated from an equation derived from multiple-attribute regression analysis of RT, DT, GR, and DEN logs. Representative P- and S-wave velocities and Poisson’s ratio are statistically computed for oil and water bearing reservoir rock, shale, and calcareous shale in each well. The averaged values are used for AVO forward modeling. Comparing the modeling results with actual seismic data limit the possible AVO interpretations. Well and seismic data are used to calibrate inverted P-wave, S-wave, Poisson’s ratio, and AVO gradient attribute data sets. AVO gradient data is used for lithofacies interpretation, P- wave data is used for acoustic impedance inversion, S-wave data is used for elastic impedance, and Poisson’s ratio data is used for detecting oil and gas. The reservoir and hydrocarbon detections are carried out sequentially. We demonstrate that the AVO attributes method can efficiently predict reservoir and hydrocarbon potential.

3D AVO analysis for identifying thin continental sandstone interbeds and deep volcanic rocks

AVO characteristics were ambiguous in thin-layer interbed sandstones and mudstones of terrestrial facies of Daqing area. Described in this paper is the 3D AVO analysis and real examples in the Sorigzhan region in the north of the Songliao basin. The AVO responses of the Fuyu sandstone reservoir, obtained through special AVO processing and interpretation as well as the AVO forward modeling under the constraints of logging data, were evident in the Sheng-81 well. It was concluded that the Fuyu sandstone reservoir presented an AVO anomaly of “amplitudes increasing with offsets”. At the same time, the deep volcanic reservoirs were ascertained and the channels of volcanic eruption were delineated at the intersections of faults in Sanzhao area, through a 3D AVO analysis, which coincided with the theory of geodynamics and tectonophysics.

3D AVO crossplotting—An effective visualization technique

Crossplots, visual representations of the relationship between two variables, are used to (a) visually identify outliers that may bias a correlation, (b) gain a visual sense for the strength of the correlation between variables, (c) detect if the relationship between variables is linear or nonlinear, (d) identify trends which may indicate multiple populations within the same data set, and (e) detect significant departures from a background trend—in other words to detect anomalies. Crossplotting is widely used in AVO analysis, because it facilitates the simultaneous and meaningful evaluation of two attributes. Generally, common lithology units and fluid types cluster together in AVO crossplot space, allowing identification of background lithology trends and anomalous off-trend aggregations that could be associated with hydrocarbons. Initially, AVO crossplotting typically used the intercept and gradient. However, in 1997, Goodway et al. used crossplots of elastic parameters (Lambda-Rho and Mu-Rho) to improve petrophysical discrimination of rock properties. Other attributes have also been used as AVO anomaly indicators (Castagna and Smith, 1994). Crossplotting appropriate pairs of attributes so that common lithologies and fluid types generally cluster together allows for straightforward interpretation. The off-trend aggregations can then be more elaborately evaluated as potential hydrocarbon indicators. This is the essence of successful AVO crossplot analysis and interpretation, all of which is based on the premise that data that are anomalous statistically are interesting geologically. This article describes attempts to extend crossplotting to three dimensions and assess any advantages that result. Ross and Sparlin (2000) made this extension by using the intercept, gradient, and inlines for a 3D seismic volume as the three dimensions on a 3D crossplot. Our procedure begins by first visualizing different combinations of the measured well-log parameters (P-velocity, S-velocity, density, porosity, and gamma ray) in two and three dimensions. Next, observed patterns are visualized and compared in the …

ABSTRACT: Developing an Exploration Tool in a Mature Trend: a 3D AVO Case Study in South Texas

Summary Successful exploration for new reservoirs in mature trends often requires trying techniques unproven in the area. The Vicksburg Formation in South Texas has been heavily explored using subsurface geology and structural mapping based on conventional seismic data. Although there is a scarcity of direct hydrocarbon indicators such as bright spots, models generated using dipole sonic data suggested that Class 2 AVO anomalies would be associated with gas reservoirs. A pilot reprocessing study demonstrated that gas reservoirs generate Class 2 AVO anomalies and that incident angles greater than about 26o are required to observe them. A large non-exclusive 3D survey was reprocessed using non-hyperbolic moveout and resulting angle stacks were analyzed. Several untested anomalies were identified, including stratigraphic traps. Wildcat drilling based on this effort has resulted in six commercial discoveries and two dry holes, a success rate significantly higher than was achieved through conventional subsurface geology and structural mapping.

ABSTRACT: Reservoir Characterization using 3D AVO and 3D Net Sand Prediction

Abstract The major issues that face us today revolve around economics. Finding hydrocarbons has been made easier with 3D data, but the proper delineation and quality of the deposit still often elude us. Application of a series of amplitude attributes indicates that applying one technology, such as Amplitude Strength, is not enough. The issues that impact the economics of a reservoir are varied and complex, therefore multiple techniques must be used to address them. Application of seismic attributes to three areas of the Gulf of Mexico, in a consistent manner, demonstrates how effective technology can be when applied properly. The three areas examined all had similar Amplitude Strength, but drilling results in the area produced varied results. The Mississippi Canyon 486 field was first examined because of the strong variation in reservoir thickness. The sixteen wells drilled in the field have a gross thickness variation of 160 feet, and net thickness variation of 105 feet. Amplitude maps over the field do not indicate this complexity, and agree very poorly with the drill results. Advances in technology, mainly in the areas of AVO, Thickness, and Rock Property estimation, offer a much clearer picture of the reservoir size. Similar technology applied to a second area correctly predicted the poor outcome of an initial well and indicated the presence of cleaner and thicker reservoir in the immediate area. A sidetrack was drilled and an economic discovery was announced. The third location also has a strong amplitude anomaly similar to the other two. The AVO signature suggests the presence of hydrocarbons, but thickness predictions indicate very low net/gross sand ratios in the reservoir. The well drilled into the anomaly was plugged and abandoned. End_of_Record - Last_Page 474-------

ABSTRACT: New Pays in an Old Play: 3D AVO Successes in the Vicksburg of South Texas

Abstract In 1995, Edge Petroleum began exploring the mature Vicksburg trend in South Texas with the aid of a speculative 3D seismic survey. Our initial exploration campaign focused on drilling untested compartments in the faulted section and met with limited success. In 1997, after the drilling of two seemingly identical structures resulted in a success and a dry hole, we conducted a seismic modeling study that indicated an AVO anomaly should be associated with the pay interval. Based on this prediction, we reprocessed the 3D data and generated angle stack volumes. Approximately 320 square miles of angle stack volumes were integrated with productive well data and analyzed using seismic interpretation software including visualization tools. This analysis showed that about half of the commercial wells were associated with AVO anomalies and about two-thirds of drilled AVO anomalies were productive. Encouraged by these findings we identified numerous prospective anomalies. In our subsequent exploration program we drilled nine AVO-supported wildcats resulting in seven commercial successes. A typical discovery encountered a high-quality reservoir with an areal extent closely matched by the associated AVO anomaly. Two of the discoveries had stratigraphic trapping components and may not have been identified without the use of AVO. Thus, the AVO tool has allowed us to explore for new gas accumulations in a mature trend with a high rate of success. Note: The full manuscript of our AVO work in the Vicksburg will be published under the title Developing an Exploration Tool in a Mature Trend: A Class 2 AVO Case Study in South Texas, in The Leading Edge, v. 17, no. 11. End_of_Record - Last_Page 239-------

3D AVO as a Primary Tool for Reservoir Characterization in the Deep Water Gulf of Mexico: ABSTRACT

3D AVO as a Primary Tool for Reservoir Characterization in the Deep Water Gulf of Mexico: ABSTRACT